A set of useful JavaScript date processing functions

Description:
These functions are very useful when dealing with any type of date fields.

They use the same format strings as the java. Text. simpledateformat class, with a few minor exceptions.

Functions include:

• Isdate ()-check if a date is valid
• Comparedates ()-See if one date is greater than another
• Formatdate ()-output a date () object in any format
• Getdatefromformat ()-parse a date string and return time in MS representing date object

// ===================================================================
// Author: Matt Kruse <matt@mattkruse.com>
// WWW: http://www.mattkruse.com/
//
// NOTICE: You may use this code for any purpose, commercial or
// private, without any further permission from the author. You may
// remove this notice from your final code if you wish, however it is
// appreciated by the author if at least my web site address is kept.
//
// You may *NOT* re-distribute this code in any way except through its
// use. That means, you can include it in your product, or your web
// site, or any other form where the code is actually being used. You
// may not put the plain javascript up on your site for download or
// include it in your javascript libraries for download. 
// If you wish to share this code with others, please just point them
// to the URL instead.
// Please DO NOT link directly to my .js files from your site. Copy
// the files to your server and use them there. Thank you.
// ===================================================================

// HISTORY
// ------------------------------------------------------------------
// May 17, 2003: Fixed bug in parseDate() for dates <1970
// March 11, 2003: Added parseDate() function
// March 11, 2003: Added "NNN" formatting option. Doesn't match up
//                 perfectly with SimpleDateFormat formats, but 
//                 backwards-compatability was required.

// ------------------------------------------------------------------
// These functions use the same 'format' strings as the 
// java.text.SimpleDateFormat class, with minor exceptions.
// The format string consists of the following abbreviations:
// 
// Field        | Full Form          | Short Form
// -------------+--------------------+-----------------------
// Year         | yyyy (4 digits)    | yy (2 digits), y (2 or 4 digits)
// Month        | MMM (name or abbr.)| MM (2 digits), M (1 or 2 digits)
//              | NNN (abbr.)        |
// Day of Month | dd (2 digits)      | d (1 or 2 digits)
// Day of Week  | EE (name)          | E (abbr)
// Hour (1-12)  | hh (2 digits)      | h (1 or 2 digits)
// Hour (0-23)  | HH (2 digits)      | H (1 or 2 digits)
// Hour (0-11)  | KK (2 digits)      | K (1 or 2 digits)
// Hour (1-24)  | kk (2 digits)      | k (1 or 2 digits)
// Minute       | mm (2 digits)      | m (1 or 2 digits)
// Second       | ss (2 digits)      | s (1 or 2 digits)
// AM/PM        | a                  |
//
// NOTE THE DIFFERENCE BETWEEN MM and mm! Month=MM, not mm!
// Examples:
//  "MMM d, y" matches: January 01, 2000
//                      Dec 1, 1900
//                      Nov 20, 00
//  "M/d/yy"   matches: 01/20/00
//                      9/2/00
//  "MMM dd, yyyy hh:mm:ssa" matches: "January 01, 2000 12:30:45AM"
// ------------------------------------------------------------------

var MONTH_NAMES=new Array('January','February','March','April','May','June','July','August','September','October','November','December','Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec');
var DAY_NAMES=new Array('Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday','Sun','Mon','Tue','Wed','Thu','Fri','Sat');
function LZ(x) {return(x<0||x>9?"":"0")+x}

// ------------------------------------------------------------------
// isDate ( date_string, format_string )
// Returns true if date string matches format of format string and
// is a valid date. Else returns false.
// It is recommended that you trim whitespace around the value before
// passing it to this function, as whitespace is NOT ignored!
// ------------------------------------------------------------------
function isDate(val,format) {
 var date=getDateFromFormat(val,format);
 if (date==0) { return false; }
 return true;
 }

// -------------------------------------------------------------------
// compareDates(date1,date1format,date2,date2format)
//   Compare two date strings to see which is greater.
//   Returns:
//   1 if date1 is greater than date2
//   0 if date2 is greater than date1 of if they are the same
//  -1 if either of the dates is in an invalid format
// -------------------------------------------------------------------
function compareDates(date1,dateformat1,date2,dateformat2) {
 var d1=getDateFromFormat(date1,dateformat1);
 var d2=getDateFromFormat(date2,dateformat2);
 if (d1==0 || d2==0) {
  return -1;
  }
 else if (d1 > d2) {
  return 1;
  }
 return 0;
 }

// ------------------------------------------------------------------
// formatDate (date_object, format)
// Returns a date in the output format specified.
// The format string uses the same abbreviations as in getDateFromFormat()
// ------------------------------------------------------------------
function formatDate(date,format) {
 format=format+"";
 var result="";
 var i_format=0;
 var c="";
 var token="";
 var y=date.getYear()+"";
 var M=date.getMonth()+1;
 var d=date.getDate();
 var E=date.getDay();
 var H=date.getHours();
 var m=date.getMinutes();
 var s=date.getSeconds();
 var yyyy,yy,MMM,MM,dd,hh,h,mm,ss,ampm,HH,H,KK,K,kk,k;
 // Convert real date parts into formatted versions
 var value=new Object();
 if (y.length < 4) {y=""+(y-0+1900);}
 value["y"]=""+y;
 value["yyyy"]=y;
 value["yy"]=y.substring(2,4);
 value["M"]=M;
 value["MM"]=LZ(M);
 value["MMM"]=MONTH_NAMES[M-1];
 value["NNN"]=MONTH_NAMES[M+11];
 value["d"]=d;
 value["dd"]=LZ(d);
 value["E"]=DAY_NAMES[E+7];
 value["EE"]=DAY_NAMES[E];
 value["H"]=H;
 value["HH"]=LZ(H);
 if (H==0){value["h"]=12;}
 else if (H>12){value["h"]=H-12;}
 else {value["h"]=H;}
 value["hh"]=LZ(value["h"]);
 if (H>11){value["K"]=H-12;} else {value["K"]=H;}
 value["k"]=H+1;
 value["KK"]=LZ(value["K"]);
 value["kk"]=LZ(value["k"]);
 if (H > 11) { value["a"]="PM"; }
 else { value["a"]="AM"; }
 value["m"]=m;
 value["mm"]=LZ(m);
 value["s"]=s;
 value["ss"]=LZ(s);
 while (i_format < format.length) {
  c=format.charAt(i_format);
  token="";
  while ((format.charAt(i_format)==c) && (i_format < format.length)) {
   token += format.charAt(i_format++);
   }
  if (value[token] != null) { result=result + value[token]; }
  else { result=result + token; }
  }
 return result;
 }

// ------------------------------------------------------------------
// Utility functions for parsing in getDateFromFormat()
// ------------------------------------------------------------------
function _isInteger(val) {
 var digits="1234567890";
 for (var i=0; i < val.length; i++) {
  if (digits.indexOf(val.charAt(i))==-1) { return false; }
  }
 return true;
 }
function _getInt(str,i,minlength,maxlength) {
 for (var x=maxlength; x>=minlength; x--) {
  var token=str.substring(i,i+x);
  if (token.length < minlength) { return null; }
  if (_isInteger(token)) { return token; }
  }
 return null;
 }

// ------------------------------------------------------------------
// getDateFromFormat( date_string , format_string )
//
// This function takes a date string and a format string. It matches
// If the date string matches the format string, it returns the 
// getTime() of the date. If it does not match, it returns 0.
// ------------------------------------------------------------------
function getDateFromFormat(val,format) {
 val=val+"";
 format=format+"";
 var i_val=0;
 var i_format=0;
 var c="";
 var token="";
 var token2="";
 var x,y;
 var now=new Date();
 var year=now.getYear();
 var month=now.getMonth()+1;
 var date=1;
 var hh=now.getHours();
 var mm=now.getMinutes();
 var ss=now.getSeconds();
 var ampm="";

 while (i_format < format.length) {
  // Get next token from format string
  c=format.charAt(i_format);
  token="";
  while ((format.charAt(i_format)==c) && (i_format < format.length)) {
   token += format.charAt(i_format++);
   }
  // Extract contents of value based on format token
  if (token=="yyyy" || token=="yy" || token=="y") {
   if (token=="yyyy") { x=4;y=4; }
   if (token=="yy")   { x=2;y=2; }
   if (token=="y")    { x=2;y=4; }
   year=_getInt(val,i_val,x,y);
   if (year==null) { return 0; }
   i_val += year.length;
   if (year.length==2) {
    if (year > 70) { year=1900+(year-0); }
    else { year=2000+(year-0); }
    }
   }
  else if (token=="MMM"||token=="NNN"){
   month=0;
   for (var i=0; i<MONTH_NAMES.length; i++) {
    var month_name=MONTH_NAMES[i];
    if (val.substring(i_val,i_val+month_name.length).toLowerCase()==month_name.toLowerCase()) {
     if (token=="MMM"||(token=="NNN"&&i>11)) {
      month=i+1;
      if (month>12) { month -= 12; }
      i_val += month_name.length;
      break;
      }
     }
    }
   if ((month < 1)||(month>12)){return 0;}
   }
  else if (token=="EE"||token=="E"){
   for (var i=0; i<DAY_NAMES.length; i++) {
    var day_name=DAY_NAMES[i];
    if (val.substring(i_val,i_val+day_name.length).toLowerCase()==day_name.toLowerCase()) {
     i_val += day_name.length;
     break;
     }
    }
   }
  else if (token=="MM"||token=="M") {
   month=_getInt(val,i_val,token.length,2);
   if(month==null||(month<1)||(month>12)){return 0;}
   i_val+=month.length;}
  else if (token=="dd"||token=="d") {
   date=_getInt(val,i_val,token.length,2);
   if(date==null||(date<1)||(date>31)){return 0;}
   i_val+=date.length;}
  else if (token=="hh"||token=="h") {
   hh=_getInt(val,i_val,token.length,2);
   if(hh==null||(hh<1)||(hh>12)){return 0;}
   i_val+=hh.length;}
  else if (token=="HH"||token=="H") {
   hh=_getInt(val,i_val,token.length,2);
   if(hh==null||(hh<0)||(hh>23)){return 0;}
   i_val+=hh.length;}
  else if (token=="KK"||token=="K") {
   hh=_getInt(val,i_val,token.length,2);
   if(hh==null||(hh<0)||(hh>11)){return 0;}
   i_val+=hh.length;}
  else if (token=="kk"||token=="k") {
   hh=_getInt(val,i_val,token.length,2);
   if(hh==null||(hh<1)||(hh>24)){return 0;}
   i_val+=hh.length;hh--;}
  else if (token=="mm"||token=="m") {
   mm=_getInt(val,i_val,token.length,2);
   if(mm==null||(mm<0)||(mm>59)){return 0;}
   i_val+=mm.length;}
  else if (token=="ss"||token=="s") {
   ss=_getInt(val,i_val,token.length,2);
   if(ss==null||(ss<0)||(ss>59)){return 0;}
   i_val+=ss.length;}
  else if (token=="a") {
   if (val.substring(i_val,i_val+2).toLowerCase()=="am") {ampm="AM";}
   else if (val.substring(i_val,i_val+2).toLowerCase()=="pm") {ampm="PM";}
   else {return 0;}
   i_val+=2;}
  else {
   if (val.substring(i_val,i_val+token.length)!=token) {return 0;}
   else {i_val+=token.length;}
   }
  }
 // If there are any trailing characters left in the value, it doesn't match
 if (i_val != val.length) { return 0; }
 // Is date valid for month?
 if (month==2) {
  // Check for leap year
  if ( ( (year%4==0)&&(year%100 != 0) ) || (year%400==0) ) { // leap year
   if (date > 29){ return 0; }
   }
  else { if (date > 28) { return 0; } }
  }
 if ((month==4)||(month==6)||(month==9)||(month==11)) {
  if (date > 30) { return 0; }
  }
 // Correct hours value
 if (hh<12 && ampm=="PM") { hh=hh-0+12; }
 else if (hh>11 && ampm=="AM") { hh-=12; }
 var newdate=new Date(year,month-1,date,hh,mm,ss);
 return newdate.getTime();
 }

// ------------------------------------------------------------------
// parseDate( date_string [, prefer_euro_format] )
//
// This function takes a date string and tries to match it to a
// number of possible date formats to get the value. It will try to
// match against the following international formats, in this order:
// y-M-d   MMM d, y   MMM d,y   y-MMM-d   d-MMM-y  MMM d
// M/d/y   M-d-y      M.d.y     MMM-d     M/d      M-d
// d/M/y   d-M-y      d.M.y     d-MMM     d/M      d-M
// A second argument may be passed to instruct the method to search
// for formats like d/M/y (european format) before M/d/y (American).
// Returns a Date object or null if no patterns match.
// ------------------------------------------------------------------
function parseDate(val) {
 var preferEuro=(arguments.length==2)?arguments[1]:false;
 generalFormats=new Array('y-M-d','MMM d, y','MMM d,y','y-MMM-d','d-MMM-y','MMM d');
 monthFirst=new Array('M/d/y','M-d-y','M.d.y','MMM-d','M/d','M-d');
 dateFirst =new Array('d/M/y','d-M-y','d.M.y','d-MMM','d/M','d-M');
 var checkList=new Array('generalFormats',preferEuro?'dateFirst':'monthFirst',preferEuro?'monthFirst':'dateFirst');
 var d=null;
 for (var i=0; i<checkList.length; i++) {
  var l=window[checkList[i]];
  for (var j=0; j<l.length; j++) {
   d=getDateFromFormat(val,l[j]);
   if (d!=0) { return new Date(d); }
   }
  }
 return null;
 }